In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, 3rd Generation Partnership Project (3GPP) Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
Today there are approximately 5 billion connectable devices in the radio communication networks. One vision for the future is that communication becomes so cheap so that network communication capabilities can be built into almost every small device or object. In the future the radio communications networks may need to serve up to 50 billion, or even 500 billion devices. The use cases that are currently being studied for a “500B vision” are: mass monitoring, e.g. of agricultural fields, energy infrastructure, and parking lots; mass controlling, e.g. of traffic lights, building automation, and lamps; object tracking, e.g. of baggage, parcels, and keys; personal health care, e.g. heart rate monitoring.
In a vision it is assumed that every small device can communicate. Most devices will be sending and/or receiving small amounts of data and for many devices the energy consumption will be the most important design limitation. For simple devices, such as small sensors, the battery might need to last for several years, since replacing the batteries more often than that will simply be prohibitively expensive. The devices need to be cheap and hence we assume that the typical device battery will have limited capacity, e.g. in the order of a standard AAA battery.
For many simple devices the energy consumption in the receiving circuits of the simple device will be dominating simply because the simple device does not transmit very often. The receiver (RX) radio front-end of the simple device is dominating in energy consumption and it is particularly expensive for the simple device to have the RX radio front-end activated. Typically the simple device in idle mode needs to activate the RX radio front-end when e.g. receiving data, listening for paging transmissions from the network, and/or performing radio measurements. During sleep mode the RX radio front-end, the RX base band, and the fine clock e.g. the local oscillator, can all be turned off and only a coarse clock that is used to wake up the simple device when the simple device need to be active. The coarse clock can be made extremely power efficient but the accuracy is typically rather poor.
The energy consumption in the radio communications network is dominated by the power amplifiers in the radio base stations. In order to reduce network energy consumption in future systems it is important to design for as much support of Discontinuous Transmission (DTX) as possible since the transmitters are then silent, which also leads to increased intervals of Discontinuous Reception (DRX). The mandatory overhead transmissions that do not relate to user plane data shall preferably transmitted only in short bursts every 10-100 ms or so, such as synchronization signal transmissions, a paging message, cell specific reference signals, master information block on a Physical Broadcast Channel (PBCH), or transmissions of higher layer system information blocks.
In order to reduce the energy consumption in the devices being in idle mode increased paging intervals is beneficial. For ultra-low power devices for Machine-to-Machine (M2M) communication the paging intervals can be measured in minutes or hours rather than seconds. Since the power required for the receiving device, also referred to as a user equipment, to have the RX radio front-end in active mode is so significant, it is important to keep the wake-up period required to receive a paging message as short as possible. Typically the RX radio front-end is active for 1-2 ms which is considered acceptable to read a paging message.
The problem is that long DRX cycles cause a significant timing uncertainty in the user equipment due to the fact that the coarse clock that is running in the idle mode is of low accuracy. Long DTX periods in the radio base stations and long DRX periods in the user equipments are wanted for the purpose of reducing energy consumption. It turns out that short wakeup periods in the user equipments are difficult to match with long DTX periods since the user equipments cannot quickly find the network timing, i.e. the synchronization clock of the network, when the network is not continuously transmitting the timing information.
In case the user equipment wakes up to read a paging message with an uncertain timing, then the period the user equipment receives transmissions needs to be long in order to capture the paging message transmitted from the network to activate the user equipment.
Today one may try and solve the problem by using shorter DRX cycles in the user equipments. This reduces the time the user equipment can be in sleep mode and hence it has the drawback of high energy consumption in the mobile terminal. Alternatively, the user equipment may keep the RX radio front-end active until it receives an overhead transmission carrying an information signal associated with the time structure in the radio base station, such as a synchronization signal transmission or the actual paging message, from the radio base station. This method increases the time the user equipment needs to have the RX radio front-end in active mode and hence it has the drawback of high energy consumption in the user equipment. Furthermore, the radio base station may transmit the information signal, such as synchronization signals, more often. This limits the applicability of network DTX in the radio base station and hence it has the drawback of high energy consumption in radio base station. Thus, the different solutions known in prior art require increased energy consumption in the radio communications network or in the user equipments.